Seeking in liquidsoap
=====================
Starting with Liquidsoap `1.0.0-beta2`, it is now possible to seek within sources! 
Not all sources support seeking though: currently, they are mostly file-based sources
such as `request.queue`, `playlist`, `request.dynamic.list` etc..

The basic function to seek within a source is `source.seek`. It has the following type:

```
(source('a),float)->float
```

The parameters are:

* The source to seek.
* The duration in seconds to seek from current position.

The function returns the duration actually seeked.

Please note that seeking is done to a position relative to the *current*
position. For instance, `source.seek(s,3.)` will seek 3 seconds forward in
source `s` and `source.seek(s,(-4.))` will seek 4 seconds backward.

Since seeking is currently only supported by request-based sources, it is recommended
to hook the function as close as possible to the original source. Here is an example
that implements a server/telnet seek function:
```liquidsoap
# A playlist source
s = playlist("/path/to/music")

# The server seeking function
def seek(t) =
  t = float_of_string(default=0.,t)
  log("Seeking #{t} sec")
  ret = source.seek(s,t)
  "Seeked #{ret} seconds."
end

# Register the function
server.register(namespace=source.id(s),
                description="Seek to a relative position \
                             in source #{source.id(s)}",
                usage="seek <duration>",
                "seek",seek)
```

Cue points
----------

Sources that support seeking can also be used to implement cue points.
The basic operator for this is `cue_cut`. Its has type:

```
(?id:string,?cue_in_metadata:string,
 ?cue_out_metadata:string,
 source(audio='#a,video='#b,midi='#c))->
    source(audio='#a,video='#b,midi='#c)
```

Its parameters are:

* `cue_in_metadata`: Metadata for cue in points, default: `"liq_cue_in"`.
* `cue_out_metadata`: Metadata for cue out points, default: `"liq_cue_out"`.
* The source to apply cue points to.

The values of cue-in and cue-out points are given in absolute
position through the source's metadata. For instance, the following
source will cue-in at 10 seconds and cue-out at 45 seconds on all its tracks:

```liquidsoap
s = playlist(prefix="annotate:liq_cue_in=\"10.\",liq_cue_out=\"45\":",
             "/path/to/music")

s = cue_cut(s)
```

As in the above example, you may use the `annotate` protocol to pass custom cue
points along with the files passed to Liquidsoap. This is particularly useful 
in combination with `request.dymanic` as an external script can build-up
the appropriate URI, including cue-points, based on information from your
own scheduling back-end.

Alternatively, you may use `metadata.map` to add those metadata. The operator
`metadata.map` supports seeking and passes it to its underlying source.


